Elena V. Levchenko

Atomic mechanisms of pure iron vitrification

It is shown on the basis of the model of iron with the Pak-Doyama paired potential of interatomic interaction in the framework of the molecular dynamics method that structural stabilization of the amorphous phase of pure iron during hardening from melt is ensured by the formation of a percolation cluster from mutually penetrating and contacting icosahedrons with atoms at vertices and centers.

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni50Al50 amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10−2 m/s. The heat of the intermixing with the formation of Ni50Al50 amorphous alloy can be estimated at approximately −0.34 eV/at. Next, the crystallization of the Ni50Al50 amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately −0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.

Shrinking kinetics by vacancy diffusion of a pure element hollow nanosphere

In this paper, shrinking via the vacancy mechanism of a hollow mono-atomic nanosphere is described. Using Gibbs–Thomson boundary conditions, an exact solution is obtained for the kinetic equation in quasi steady-state at the linear approximation. Collapse time as a function of the geometrical size of a hollow nanosphere is found. An extension to hollow binary alloy nanospheres is also made. Previous Monte Carlo simulations of this problem are discussed.

Kinetics of Isothermal Nucleation in a Supercooled Iron Melt

An isothermal kinetic diagram for the beginning of homogeneous nucleation is constructed using the molecular-dynamics model of an instantaneously supercooled iron melt near the icosahedral percolation transition temperature identified with the glass transition temperature Tg. This diagram is compared with the theoretical one calculated using quantitative information obtained by analyzing the kinetics of the initial stage of growth of supercritical nuclei at temperatures higher than Tg. A satisfactory coincidence of the theoretical curve with computer simulation data at temperatures higher than Tg and substantial disagreement with these data below Tg, where crystallization is necessarily preceded by the formation of an icosahedral percolation cluster, demonstrate the substantive influence of an icosahedral substructure on the nucleation rate predicted by the classical theory.

Interdiffusion and surface-sandwich ordering in initial Ni-core–Pd-shell nanoparticle

Using molecular dynamics simulation ( approximately 1 mus) in combination with the embedded atom method we have investigated interdiffusion and structural transformations at 1000 K in an initial core-shell nanoparticle (diameter approximately 4.5 nm). This starting particle has the f.c.c. structure in which a core of Ni atoms ( approximately 34%) is surrounded by a shell of Pd atoms ( approximately 66%). It is found that in such nanoparticles reactive diffusion accompanying nucleation and growth of a Pd(2)Ni ordering surface-sandwich structure takes place. In this structure, the Ni atoms mostly accumulate in a layer just below the surface and, at the same time, are located in the centres of interpenetrating icosahedra to generate a subsurface shell as a Kagomé net. Meanwhile, the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from the inside and outside as well as being located in the core of the nanoparticle forming (according to the alloy composition) a Pd-rich solid solution with the remaining Ni atoms. The total atomic fraction involved in building up the surface-sandwich shell of the nanoparticle in the form of the Ni Kagomé net layer covered on both side by Pd atoms is estimated at approximately 70%. These findings open up a range of opportunities for the experimental synthesis and study of new kinds of Pd-Ni nanostructures exhibiting Pd(2)Ni surface-sandwich ordering along with properties that may differ significantly from the corresponding bulk Pd-Ni alloys. Some of these opportunities are discussed.

Shrinking kinetics by vacancy diffusion of hollow binary alloy nanospheres driven by the Gibbs–Thomson effect

The general treatment of the Gibbs–Thomson effect for a hollow nanosphere is presented. It allows for a vacancy composition profile across the nanoshell to be defined by a continuously decreasing function as well as by a continuous function with a minimum. The range for the controlling parameter of the vacancy motion within a binary alloy nanoshell is determined in terms of the phenomenological coefficients as well as the (measurable) tracer diffusion coefficients ( ) of the atomic components. On the basis of a theoretical description and kinetic Monte Carlo simulations, it is demonstrated that for a hollow random binary alloy nanosphere with an equi-atomic (initially homogeneous) composition and neglecting the radial dependence of vacancy formation free energy, the controlling parameter of the shrinking rate in the limiting case can be estimated with reasonable accuracy as the geometric mean of the tracer diffusion coefficients of the atomic components.

Thermotransport in binary system: case study on Ni50Al50 melt

The formalism of thermotransport in a binary system is analysed. Focus is put on a detailed consideration of the heat of transport parameter characterizing diffusion driven by a temperature gradient. We introduce the reduced heat of transport parameter , which characterizes part of the interdiffusion flux that is proportional to the temperature gradient. In an isothermal system represents the reduced heat flow (pure heat conduction) consequent upon unit interdiffusion flux. It is demonstrated that is independent of reference frame and is useful in a practical way for direct comparison of simulation and experimental data from different sources obtained in different reference frames. In the case study of the liquid Ni50Al50 alloy, we use equilibrium molecular dynamics simulations in conjunction with the Green–Kubo formalism to evaluate the heat transport properties of the model within the temperature range of 1500–4000 K. Our results predict that in the presence of a temperature gradient Ni tends to diffuse from the cold end to the hot end whilst Al tends to diffuse from the hot end to the cold end.

The influence of the icosahedral percolation transition in supercooled liquid iron on the diffusion mobility of atoms

The paper develops concepts of the structure of pure amorphous metals and atomic mechanisms of its formation. It is shown that a stable percolation cluster of interpenetrating and contacting icosahedra whose vertices and centers are occupied by atoms is formed under the conditions of isothermal annealing of instantaneously supercooled iron melt only below the critical temperature ∼1180 K identified with the glass transition temperature. The duration of isothermal annealing up to the formation of the icosahedral percolation cluster does not exceed ∼1.5 × 10−11 s at 900–1180 K. The time of the beginning of homogeneous nucleation was found to be minimum at the critical temperature above which stable icosahedral percolation cluster did not form. Arguments are provided in favor of the assumption that the formation of icosahedral percolation cluster interferes with the beginning of crystallization. A quantitative model is suggested to describe the diffusion mobility of atoms in metallic glasses. In this model, the mean-square displacement of atoms is represented as the sum of the contributions of the linear (Einstein) and logarithmic components. The latter appears because of irreversible structural relaxation. The icosahedral percolation transition was shown to change the activation parameters of the model jumpwise.

Molecular dynamics prediction of phonon-mediated thermal conductivity of f.c.c. Cu

The phonon-mediated thermal conductivity of f.c.c. Cu is investigated in detail in the temperature range 40–1300 K. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green–Kubo formalism and one of the most reliable embedded-atom method potentials for Cu. It is found that the temporal decay of the heat current autocorrelation function (HCACF) of the Cu model at low and intermediate temperatures demonstrate a more complex behaviour than the two-stage decay observed previously for the f.c.c. Ar model. After the first stage of decay, it demonstrates a peak in the temperature range 40–800 K. A decomposition model of the HCACF is introduced. In the framework of that model we demonstrate that a classical description of the phonon thermal transport in the Cu model can be used down to around one quarter of the Debye temperature (about 90 K). Also, we find that above 300 K the thermal conductivity of the Cu model varies with temperature more rapidly than , following an exponent close to −1.4 in agreement with previous calculations on the Ar model. Phonon thermal conductivity of Cu is found to be about one order of magnitude higher than Ar. The phonon contribution to the total thermal conductivity of Cu can be estimated to be about 0.5% at 1300 K and about 10% at 90 K.

Molecular dynamics study of phonon-mediated thermal transport in a Ni50Al50melt: case analysis of the influence of the process on the kinetics of solidification

The phonon-mediated contribution to the thermal transport properties of liquid NiAl alloy is investigated in detail over a wide temperature range. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green–Kubo formalism and one of the most reliable embedded-atom method potentials for the intermetallic alloy. The phonon-mediated contribution to the thermal conductivity of the liquid alloy is calculated at equilibrium as well as for the steady state. The relative magnitude of the thermal conductivity decrease induced by the transition to the steady state is estimated to be less than 2% below 2000 K and less than 1% at 3000 and 4000 K. It is also found that the phonon-mediated contribution to the thermal conductivity of the liquid alloy can be accurately estimated (well within 1%) on the basis of an approximation which invokes the straightforwardly accessible microscopic expression for the total heat flux without demanding calculations of the partial enthalpies needed for the precise evolution of the reduced heat flux (pure heat conduction). On the basis of these calculations, the correspondence between the experimentally observed and modelled kinetics of solidification due to a difference in thermal conductivity is discussed.